Technical Field
This disclosure relates to the field of additive manufacturing. This disclosure also relates to the field of 3-D printing. This disclosure particularly relates to the fabrication of thermoelectric devices, such as thermoelectric generators. This disclosure also relates to methods for employing additive-manufacturing systems and/or devices for producing useful materials and/or objects, including thermoelectric devices, such as thermoelectric generators.
Description of Related Art
There is currently considerable research and development effort being directed at the development of devices which are capable of generating electricity from low-level or waste energy sources. Among these devices, which are sometimes known as thermoelectric generators, are models in which a modest temperature difference can be used to generate an electric current. Such devices rely on the incorporation of special substances, called thermoelectric materials, which can generate electricity from low-level temperature differences. These types of devices can be used to generate power in a wide variety of applications, and are expected to play a significant role in addressing the global energy crisis.
In addition to the challenge of developing efficient and cost-effective thermoelectric materials, another impediment to the widespread applicability of thermoelectric generators is the cost and complexity of assembling useful devices which incorporate them. Most such devices are currently assembled by hand using a variety of different materials, which not only makes them cost-prohibitive but also introduces unnecessary complication, lowers reliability and reproducibility, and degrades their performance. Clearly, better fabrication methods are needed to facilitate the widespread acceptance of thermoelectric generators.
Additive manufacturing refers to a family of technologies for building three-dimensional (3-D) solid or partially-solid objects by sequentially or simultaneously depositing layers of materials according to a design produced using a computer-aided design (CAD) software application. The family of additive manufacturing (AM) techniques has proven useful for the rapid production of complex prototypes as well as the manufacture of complex and complicated objects, and is especially well-suited to fabricating complicated objects in a rapid and cost-effective manner.
Additive manufacturing can be used to create highly-customized complex parts and products that are difficult or impossible to manufacture using traditional technologies. This technology can also be used to rapidly create prototype objects which could take much longer to produce by other means. This technology can also be used to create objects at a lower cost than they could be produced using other means.
One especially useful form of additive manufacturing is known as 3-D printing. In 3-D printing, multiple layers of material (referred to generally as the ‘build material’) are laid down successively to produce a three-dimensional object.
There are several major 3-D printing technologies differing mainly in the way successive layers are built to create the final 3-D object. Some methods use melting or softening and deposition of the build material to produce the layers of the growing object. For example, fused-deposition modeling (FDM) works by extruding melted plastic or metal, often supplied in the form of filaments or wires, through an extrusion nozzle to form the successive layers. On the other hand, selective laser sintering (SLS) works by laying down a thin layer of powdered metal, plastic, ceramic, or glass and then sintering the intended cross-sectional area of each layer to produce the desired object. Powder printing works similarly, except that the layers of powdered materials which are laid down are then printed over using a technology such as an ink-jet printer to create the cross-sectional image of the desired object. Stereolithography (or stereolithographic assembly, SLA) is based on photocuring (polymerizing) liquid materials such as polymer resins by applying external energy sources such as ultraviolet (UV) or visible light or electron-beam irradiation to produce each successive layer of a solid object. Each of these additive manufacturing techniques has important applications in the fields of prototyping and manufacturing.
Two relevant publications which detail current research efforts on thermoelectric materials and thermoelectric generators include G. J. Snyder and E. Toberer, Nature Materials 7, 105 (2008) and P. Sheng, Y. Sun, F. Jiao, C. Di, W. Xu, and D. Zhu, Synthetic Metals 193, 1-7 (2014). The entire content of these publications is incorporated herein by reference.
The illustration in FIG. 1 is taken from Figure B1 in G. J. Snyder and E. Toberer, Nature Materials 7, 105 (2008), and shows a schematic representation of a thermoelectric generator. The outer, non-electrically conducting substrates 10a, 10b are shown in gray; these are typically made from materials such as poly(dimethylsiloxane) (PDMS) or a poly(imide) such as Kapton®, which are familiar commercial materials to those skilled in the art. A series of thermoelectric elements 12a, 12b, or legs, is represented as vertical members with square cross sections. A series of metal interconnects 14a provides thermal connections between pairs of legs at the top of the generator, while another set of metal interconnects 14b provides electrical connections between pairs of legs at the bottom of the generator. These connections alternate, so that the thermal connections occur between different legs than do the electrical connections. The flow of electrical current is thus up through an n-type leg, across the thermal connector, down through an p-type leg, across the electrical connector, and then up again through the next n-type leg, and so on until it reaches the distal terminal and exits the thermoelectric generator.
Although additive manufacturing would appear to provide an excellent process for both prototyping and manufacturing thermoelectric generators, three major impediments have thus far hindered this approach. First, thermoelectric generators are generally constructed from a heterogeneous set of materials, including polymers, carbon forms, metals, plastics, and other materials, which generally cannot be 3-D printed using any one 3-D printer. Thus, the assembly of thermoelectric generators has been restricted to manual methods. The other impediment has been that it has not been possible to 3-D print the types of thermally- and electrically-conducting materials which are required for the construction of efficient thermoelectric generators. Such materials have demanding properties which require very specific operational parameters, and have thus far proven impossible to 3-D print. The third impediment is specific to stereolithographic assembly (SLA), a preferred method of 3-D printing due to its many advantageous features. A drawback of SLA, however, is that it is generally restricted to a single material. That is, only one type of material can be 3-D printed at a time. Overcoming these three impediments would be expected to lead to the capability to mass-produce thermoelectric generators for a wide variety of applications. This capability, in turn, would enable a wide array or new and novel products with potential applications in home heating, automotive power, industrial generation, aerospace operations, marine environments, and widely-distributed power generation, among many other applications. The electricity generated by these devices could be used to power electronic devices such as energy-storage devices, communications devices, medical devices, ballistic monitors, aircraft and aerospace vehicles, as well as numerous other items.
Thus, the ability to combine efficient thermoelectric materials with a more-efficient and manufacturable design, reduced numbers of materials, and a practical electrically-conducting 3-D printing process would enable a large family of new and novel thermoelectric generators with applicability to a wide variety of fields and industries.